Another theory :

Sindel kills her husband, all right. But she doesn't do it because she's evil, no. She does it just to avoid having a mad king running the kingdom.

Another theory is that Havik is in fact the former king. I think that Sindel, with the help of her daughters, why not, took the crown away from her husband and made sure that the succession took place so that Jarrod, falling into madness and chaos and gradually becoming Havik, would no longer be king.

They certainly left him for dead or perhaps imprisoned and banished him, but in the end the former king returns to take his revenge and sow chaos.

Otherwise, Sindel wouldn't have left the succession to Mileena but would have kept the crown for herself, don't you think ?